The invention is concerned with intentionally and desirably varying the viscosity of a fluid or semi-fluid so that it may at one time possess a high viscosity value, e.g., high capacity for suspending particulate materials, and at a subsequent time have a reduced viscosity value, e.g., lower capacity for suspending such materials. There are a number of industrial operations wherein it is desirable that the capacity of a liquid to suspend and later to drop out or permit settling of a particulate material suspending therein be controlled.
Exemplary of the field of use of controlled viscosity fluids includes applications as drilling fluids, as water flooding fluids, as carrier fluids in sand control operations such as emplacement of a gravel pack or consolidated gravel pack, and the like, particularly where low fluid loss and/or particulate transport properties are desired. Such fluids are also employed to transport solids through pipelines, and the like.
Of particular interest to the invention, and an area where need for improved viscosity control fluids exists, is in the practice of fracturing subterranean formations. In these applications, characteristically a fracturing fluid (usually a liquid, although some gas may be present), having granular insoluble particulate material suspended or slurried therein, is injected down a well penetrating the formation and forced back into the formation (employing the height of the hydraulic head of the liquid to provide pressure and hence commonly known as hydraulic fracturing). Through the instrumentality of such operation, the formation is caused to crack or fracture thereby effectuating improved communication between the recoverable fluid (e.g., oil, gas, water, etc.) in the formation and the well. Fracturing is customarily conducted by employing oil, water, (sometimes in a thickened or gelled state) or an emulsion thereof which has suspended therein particles which are substantially insoluble in the liquid and the fluids of the formation, at least a portion of the particles thereby being forced to lodge in the fractures created, thus propping open the fractures when the fracturing pressures are subsequently released and the well put back into production.
State of the art variable viscosity fracturing fluids or water-gels are frequently less than satisfactory for application to the special case of relatively low temperature oil and gas reservoirs (defined as from about 50.degree. to 125.degree. F.). For example, as the gelling agent or viscosity builder concentration is increased, the water-gel or fracturing fluid requires extended times to effect a complete break. The delay caused by the inability to effect a rapid and complete break under relatively cool reservoir conditions, disadvantageously extends the well clean-up time, i.e., retraction of the water-gel or fracturing fluid from the well bore, and in some cases inhibits a satisfactory clean-up operation. Consequently, the art has tended to favor modified techniques which are designed using light viscosity fracturing fluids to circumvent or partially circumvent the unsuitable nature of the more viscous, frequently crosslinked gels or fluids. Such modified techniques produce the undesirable result of a lowering of the permissible proppant concentration, coupled with the need for substantially increased pumping rates. Recent studies have shown that the importance of high total proppant volume in good stimulation treatments and the importance of controlling pumping rates for good fracture height control. Accordingly, there exists a clear need for improved fluids possessing improved low temperature breaking characteristics, and which are thus more suitable for the above and similar applications.